Any reason not to go for the low-cost foam?

Let's say it again: the better stuff costs more money. There are many possible
urethane foam formulations all of which vary wildly in their performance and
properties.

does cheap foam break down over time or something?

Of course it does. Heat and moisture will cause some formulations to revert
partially to their unreacted state, i.e., they turn to goop. Others will
oxidize, turn to brittle chunks or powdery crumbles. The idea that there is
only one kind of foam is way wrong, so if your project needs long term survival
don't waste your time on marginal projects. HTH

Joe